Spread spectrum communications are commonly used in cellular networks that provide wireless service to mobile stations, such as wireless telephones. Such cellular networks may operate, for example, in accordance with IS-95 CDMA or cdma2000 standards.
The signals transmitted by base transceiver stations (BTSs) in a spread spectrum communication system are spread by a pseudonoise (PN) sequence. For example, in accordance with IS-95 CDMA standards, the pilot signal transmitted by a BTS is spread by a “short” PN sequence at a chipping rate of 1.2288 MHz, and the “short” PN sequence repeats itself every 26.67 milliseconds. With this chipping rate, one “chip” is approximately 0.8138 microseconds.
Each pilot signal transmitted by a spread spectrum communication system may be spread by the same short PN sequence but with a different phase or “PN offset.” In this regard, the beginning of a PN sequence used to spread a signal may occur at a particular time offset relative to a reference time (such as the beginning of every even second), according to the system time that is used by the spread spectrum communication system. The “PN offset” of the signal may then correspond to this particular time offset.
In accordance with IS-95 CDMA standards, each PN offset is defined by an index that is an integer in the range of 0 through 511. The actual PN offset is then found by multiplying its index by 64 chips. Thus, a PN offset of “0” means that the PN sequence begins at the reference time. A PN offset of “1” means that the PN sequence begins 64 chips after the reference time. A PN offset of “2” means that the PN sequence begins 128 chips after the reference time, etc. In this way, different pilot signals may be distinguished by their PN offsets.
As noted above, the PN offset of a pilot signal is defined with respect to a reference time according to the system's time. However, when the pilot signal is received by a mobile station, the mobile station may measure a different PN offset. The difference between the PN offset transmitted by a BTS (the nominal PN offset) and the PN offset measured by the mobile station (the measured PN offset) can arise for at least two reasons: (1) there may be a propagation delay between when the pilot signal is transmitted by the BTS and when it is received by the mobile station; and (2) the mobile station's time may not be perfectly synchronized to the system's time.
The propagation delay can be a function of the distance between the BTS and the mobile station. For example, a spread spectrum signal will travel approximately 244 meters in one chip (assuming that one chip is 0.8138 microseconds). Thus, it takes about 6.6 chips for a spread spectrum signal to travel one mile. This means that if the mobile station is trying to detect a pilot signal from a target BTS that is one mile away, the mobile station will measure a PN offset that is 6.6 chips greater than the nominal PN offset of the target BTS's pilot signal (provided that the mobile station's time is perfectly synchronized with the system's time).
However, the mobile station's time may not be perfectly synchronized with the system's time. Typically, the mobile station seeks to synchronize its time with the system's time based on system time information transmitted by a source BTS in the spread spectrum communication system. However, there is a propagation delay between when the source BTS transmits the system time information and when the mobile station receives it, due to the distance between the source BTS and the mobile station. The propagation delay associated with the system time information causes the mobile station's time to lag the system's time, thereby making the PN offsets of pilot signals received by the mobile station appear to be less than they would otherwise.
The net effect of the propagation delays will depend on how, DS, the distance between the mobile station and the source BTS (the source of the system time information used by the mobile station), compares to DT, the distance between the mobile station and the target BTS (the BTS that transmits the target pilot signal being sought by the mobile station). If DS is equal to DT, then the measured PN offset of the target pilot signal may be equal to its nominal PN offset. If DS is less than DT, then the measured PN offset of the target pilot signal may be greater than its nominal PN offset. If DS is greater than DT, then the measured PN offset of the target pilot signal may be less than its nominal PN offset.
Because the PN offset of a pilot signal that is measured by a mobile station can be either greater than or less than the nominal PN offset of the pilot signal, a mobile station may use a search window centered on the nominal PN offset to search for the pilot signal. The search window typically has a width (in chips) that is specified by a width parameter that the mobile station receives from the spread spectrum communication system. In this way, the mobile station may find a target pilot signal so long as the target pilot signal's PN offset at the mobile station is within the range of PN offsets specified by the search window.